Cracking analysis on joint lug of aluminum alloy framework of an airplane

An aluminum alloy joint lug of the framework of an airplane was found to have a crack near the bolt hole after serving for a period. To find out the failure mode and cause, macro and micro observation, microstructure examination, EDS analysis, chemical composition analysis and dimensional check were carried out. The results show that the failure mode of the joint lug is stress corrosion cracking under the co-effect of tensile stress and corrosive environment. The cracking of the surface anodic oxidation film near the bolt hole and the exposure of part of the bolt hole inner surface to air are the main causes for the stress corrosion cracking. In addition, the outer diameter of the steel sleeve inside the bolt hole exceeded the design requirement, which resulted in greater tensile stress near the bolt hole and promoted the stress corrosion cracking. The manufacturing procedure should be adjusted to avoid the cracking of anodic oxidation film, the uncovered part of the inner surface of the bolt hole should be covered, and the outer diameter of the steel inside the bolt hole should be controlled properly to meet the design requirement, so that stress corrosion cracking near the bolt hole can be avoided.

► The failure mode of a joint lug is stress corrosion cracking.
► The cracking of the inner surface anodic oxidation film is the main cause.
► Some prevention measures for such failure were proposed.